The Effects of Chlamydia on Koala Populations


Studies examining the high prevalence of disease in koala populations have been recognised for well over a century. However, only within the previous decade have the significance of diseases as a major contributor to species decline been properly acknowledged.


Chlamydia (Chlamydia Pecorum) is a sexually transmitted bacterial infection. Chylamydia pecorum which originated from ruminants is the strain of chlamydia that effects koalas. The pathogen is a key factor in the decline of koalas and is one of the leading causes of death of the Australian marsupial. The infection is extremely common affecting most mainland koalas and many island populations, plaguing populations throughout their distribution. Over the previous two decades, a documented decline of up to 80% of koala populations has been recorded due to chlamydia infections and related illnesses. Some wild populations in Australia have been tested at a staggering 100 per cent infection rate. As well as being detected in Queensland, New South Wales, Victoria, and South Australia, the disease has been seen in captive koalas globally. Surveys of patient records from treatment facilities found Chlamydia had the highest morality rate of all diseases affecting koalas.


Apart from spreading through breeding koalas the disease can also be transmitted to joeys in the pouch once they begin weaning and ingest their mother’s pap, meaning the spread of the disease is extremely hard to control. Due to habitat destruction and fragmentation to accommodate the expanding urbanisation of Australia, there is less available territory for koalas. More isolated groups mean it becomes increasingly harder to find mates that are healthy and genetically diverse.

Looking at the effects of the disease indicates why mortality rates are so high. Symptoms of the infection include kerato-conjunctivitis, inflammation of the conjunctival tissue and in chronic infections left untreated ocular scarring and blindness can occur. The survival chances of a blind koala in the wild is not optimistic. Pneumonia and in severe cases death are also common outcomes of the infection. Urinary tract infections (UTI) are another common symptom, in females chronic UTI’s can lead to the development of pelvic inflammatory disease and ovarian cysts, resulting in infertility. Some states do prohibit the release of infertile koalas back into the wild and these are usually placed into captive management to live out their days, this leaves an ever-dwindling supply of genetically healthy breeding females in the wild.


The infection itself and some of its symptoms like infertility or pneumonia are not easily detected by observation alone, meaning sometimes infections are not identified until the symptoms have become acute and chances of survival are lower. There are some signs that can be easily recognized. UTI can cause a loss of bladder control, and a visible brown discolouration on the bottom known as ‘wet bottom’ this can be used to identify the infection. Conjunctivitis can present as red/pink swollen eyes. If the infection has already caused blindness, a clouding of the eyes and behavioural changes may be used to identify the infection. Blindness could present with an inability to navigate their surroundings, slower moving, ground dwelling individuals that are unresponsive to human or predator presence.


Despite being actively researched at many universities the pathogen itself and its associated disease pathogenesis is still relatively poorly understood. This information is critical for the organisations trying to make targeted treatment strategies. For example severe disease is more common in northern koalas (Qld and NSW) than in southern koalas (Vic and SA). Severe cystitis, kerato- conjunctivitis and active reproductive tract disease is common in northern koalas, and very uncommon in southern koalas. Although cystic change and fibrosis of the reproductive tract leading to infertility is common in southern koalas, the severe debilitating pathology seen commonly in northern koalas is not and why is not yet fully understood making it harder for management plans.


So why can the infections simply not be treated with a course of antibiotics like in humans? Antibiotics are the main treatment used for chlamydia, but problem here lies in their high failure rates and the adverse reactions to the treatment, many koalas lose weight and even die whilst receiving treatment. The gut dysbiosis outcomes associated with antibiotics are of significant concern. Gut microbes that allow koalas to eat eucalyptus are altered during treatment with antibiotics. The koala is known as a specialist folivore having adaptations to the gastrointestinal microbiome in response to its exclusive diet of eucalyptus leaves. Scientists found there was "a strong correlation" between the composition of a koala’s gut flora and its prognosis for survival after treatment with antibiotics.


The good news is the research into safer treatment methods are being conducted, a comparative study of the two antibiotics that are most commonly given to koalas with chlamydia (chloramphenicol and doxycycline) concluded doxycycline was a more reliable treatment for defeating infection. Rosie Booth, director of Australia Zoo Wildlife Hospital stated animals were more prone to develop diarrhoea yeast infections, depression, dehydration and bone-marrow dysfunction whilst on chloramphenicol. By comparison, koalas given doxycycline were less likely to present harmful side effects. Additionally, there are some alternative non-antibiotic treatments such as adding probiotics to restore the healthy bacteria and faecal transplants where faeces from a healthy koala is provided to restore good bacteria. A vaccine has been under development for several years has been doing well in clinical trials and researchers are hopeful it will replace antibiotics in mild ocular Chlamydia cases. Although further trials and clinical research is still required, if successful it may provide an additional tool for control of chlamydial disease at the individual and population level.


A significant investment in research time and funding are required to tackle the hugely detrimental effect chlamydia outbreaks are having on koala populations. To date, the level of funding, and thus our lever of understanding, has been poor, given the iconic status of the animal and the magnitude of the threat facing its continued existence in the wild. Looking at a comparison to the government response to the Tasmanian devil facial tumour disease (DFTD) $22 million of government funding has been provided to date, the response to the threat of koala disease has been minimal. A paper by the Australian Wildlife Hospital looked at some of the potential reasons why, they concluded:

  1. That disease in koalas has been recognised for years

  2. That DFTD causes dramatic and overt pathology, and has spread rapidly; in contrast, KoRV-associated diseases such as Chlamydia is insidious and often overlooked even with veterinary assessment.

  3. Severe habitat impacts on remnant koala populations are masking the impacts of KoRV and Chlamydia.

  4. Regulatory authorities in Queensland and New South Wales are largely ignorant of the threat, mainly due to the poor level of veterinary support and disease surveillance provided to key koala rehabilitation centres.

  5. The koala is still geographically widespread and in some areas even considered “overabundant”

It is important to reiterate that the urgent need for further disease research (and the funding to support it) it is just as vital to the future survival of the species as habitat protection and restoration. Experts at the Australian Wildlife hospital believe that if diseases in koalas do not begin to receive the same attention and funding that habitat loss receives it will be a recipe for disaster. Diseases such as chlamydia are a critical threat pushing the decline and extinction of the species that needs to be addressed in population modelling and conservation planning.

Written By: Sara-Louise McCracken - Student @TISOVN (Advanced Certificate In Australian Wildlife Studies)

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